Delamination of PB-Free Flip Chip Underfill During 2nd Level Interconnect Reflow

As the need for the use of Pb-free solder in electronic packaging fields increase, many researches have been conducted to ensure the reliability of the new solder systems. Currently, Sn-based solders are generally accepted as the most promising Pb-free solders. Most of Sn-based solders (e.g. Sn/Ag, Sn/Cu, Sn/Ag/Cu) have similar melting temperatures between 220 and 240. This transition forces electronics industry to lose soldering hierarchy between the first and second level packaging. Specifically, the first level interconnect (flip chip) will be melt again during second level interconnect (BGA) reflow which allows up to 260. In the mean time, the first level interconnect, which is encapsulated by underfill material, experiences phase change from solid to liquid. Similar to most of metals, Sn-based Pb-free solders have volume expansion as much as 4.5% during the phase change. It puts hydrostatic pressure on the surrounding materials such as underfill encapsulation, chip, and substrate and ultimately delaminate the interfaces or crack the underfill material. The pressurized solder is interjected through the failure and make short circuit.